Split-can dipole antenna for an implantable medical device

ABSTRACT

An apparatus and method for enabling far-field radio-frequency communications with an implantable medical device utilizing the device housing as an antenna. Such radio-frequency communications can take place over much greater distances than with inductively coupled antennas.

FIELD OF THE INVENTION

[0001] This invention pertains to implantable medical devices such ascardiac pacemakers and implantable cardioverter/defibrillators. Inparticular, the invention relates to an apparatus and method forenabling radio-frequency telemetry in such devices.

BACKGROUND

[0002] Implantable medical devices, including cardiac rhythm managementdevices such as pacemakers and implantable cardioverter/defibrillators,typically have the capability to communicate data with a device calledan external programmer via a radio-frequency telemetry link. A clinicianmay use such an external programmer to program the operating parametersof an implanted medical device. For example, the pacing mode and otheroperating characteristics of a pacemaker are typically modified afterimplantation in this manner. Modern implantable devices also include thecapability for bidirectional communication so that information can betransmitted to the programmer from the implanted device. Among the datawhich may typically be telemetered from an implantable device arevarious operating parameters and physiological data, the latter eithercollected in real-time or stored from previous monitoring operations.

[0003] Telemetry systems for implantable medical devices utilizeradio-frequency energy to enable bidirectional communication between theimplantable device and an external programmer. An exemplary telemetrysystem for an external programmer and a cardiac pacemaker is describedin U.S. Pat. No. 4,562,841, issued to Brockway et al. and assigned toCardiac Pacemakers, Inc., the disclosure of which is incorporated hereinby reference. A radio-frequency carrier is modulated with digitalinformation, typically by amplitude shift keying where the presence orabsence of pulses in the signal constitute binary symbols or bits. Theexternal programmer transmits and receives the radio signal with anantenna incorporated into a wand which can be positioned in proximity tothe implanted device. The implantable device also generates and receivesthe radio signal by means of an antenna, typically formed by a wire coilwrapped around the periphery of the inside of the device casing.

[0004] In previous telemetry systems, the implantable device and theexternal programmer communicate by generating and sensing a modulatedelectromagnetic field in the near-field region with the antennas of therespective devices inductively coupled together. The wand must thereforebe in close proximity to the implantable device, typically within a fewinches, in order for communications to take place. This requirement isan inconvenience for a clinician and limits the situations in whichtelemetry can take place.

SUMMARY OF THE INVENTION

[0005] The present invention is an apparatus and method for enablingcommunications with an implantable medical device utilizing far-fieldelectromagnetic radiation. Using far-field radiation allowscommunications over much greater distances than with inductively coupledantennas. In accordance with the invention, separate conductive portionsof a housing for the implantable device act as a dipole antenna forradiating and receiving far-field radio-frequency radiation modulatedwith telemetry data. The antenna is dimensioned such that a substantialportion of the radio-frequency energy delivered to it at a specifiedfrequency by a transmitter in the implantable device is emitted asfar-field electromagnetic radiation. A tuning circuit may be used totune the antenna by optimizing its impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 illustrates one embodiment of a split-can dipole antenna.

[0007]FIG. 2 illustrates an alternate embodiment of a split-can dipoleantenna with the device header separating the two housing portions.

[0008]FIG. 3 is a block diagram of the components of an exemplarycardiac rhythm management device.

DETAILED DESCRIPTION

[0009] As noted above, conventional radio-frequency (RF) telemetrysystems used for implantable medical devices such as cardiac pacemakersutilize inductive coupling between the antennas of the implantabledevice and an external programmer in order to transmit and receive RFsignals. Because the induction field produced by a transmitting antennafalls off rapidly with distance, such systems require close proximitybetween the implantable device and a wand antenna of the externalprogrammer in order to work properly, usually on the order of a fewinches. The present invention, on the other hand, is an apparatus andmethod for enabling telemetry with an implantable medical deviceutilizing far-field radiation. Communication using far-field radiationcan take place over much greater distances which makes it moreconvenient to use an external programmer. Also, the increasedcommunication range makes possible other applications of the telemetrysystem such as remote monitoring of patients and communication withother types of external devices.

[0010] A time-varying electrical current flowing in an antenna producesa corresponding electromagnetic field configuration that propagatesthrough space in the form of electromagnetic waves. The total fieldconfiguration produced by an antenna can be decomposed into a far-fieldcomponent, where the magnitudes of the electric and magnetic fields varyinversely with distance from the antenna, and a near-field componentwith field magnitudes varying inversely with higher powers of thedistance. The field configuration in the immediate vicinity of theantenna is primarily due to the near-field component, also known as theinduction field, while the field configuration at greater distances isdue solely to the far-field component, also known as the radiationfield. The near-field is a reactive field in which energy is stored andretrieved but results in no net energy outflow from the antenna unless aload is present in the field, coupled either inductively or capacitivelyto the antenna. The far-field, on the other hand, is a radiating fieldthat carries energy away from the antenna regardless of the presence ofa load in the field. This energy loss appears to a circuit driving theantenna as a resistive impedance which is known as the radiationresistance. If the frequency of the RF energy used to drive an antennais such that the wavelength of electromagnetic waves propagating thereinis much greater than the length of the antenna, a negligible far-fieldcomponent is produced. In order for a substantial portion of the energydelivered to the antenna to be emitted as far-field radiation, thewavelength of the driving signal should not be very much larger than thelength of the antenna.

[0011] A dipole antenna is made of two lengths of metal, usuallyarranged end to end with the cable from a transmitter/receiver feedingeach length of the dipole in the middle. An efficiently radiatingresonant structure is formed if each length of metal in the dipole is aquarter-wavelength long, so that the combined length of the dipole fromend to end is a half-wavelength. A wire antenna for an implantablemedical device capable of emitting far-field radiation, however, mayrequire special implantation procedures and may also be broken ordeformed as a patient moves resulting in de-tuning. In accordance withthe present invention, a dipole antenna for an implantable medicaldevice is formed by separate conductive portions of the device housingor can, referred to herein as a split-can dipole antenna. In oneembodiment, the conductive housing is split into two halves separated byan insulating dielectric material, with each half connected totransmitting/receiving circuitry contained within one of the housingportions. Unlike wire antennas, a split-can dipole antenna does notrequire any special implantation procedures and is a rigid structurewhich is resistant to breakage or deformation.

[0012] An antenna most efficiently radiates energy if the length of theantenna is an integral number of half-wavelengths of the driving signal.A half-wave dipole antenna, for example, is a center-driven conductorwhich has a length equal to half the wavelength of the driving signal.The natural tuning of a split-can dipole antenna depends, of course onthe device size. For example, a typical lengthwise dimension of animplantable cardiac rhythm management device may be about 6.8 cm, whichcorresponds to a half wavelength of a 2.2 GHz carrier frequency. If eachhalf of a split-can dipole antenna is 3.4 cm, then the antenna is ahalf-wavelength dipole at that carrier frequency. For medical deviceapplications, carrier frequencies between 300 MHz and 1 GHz are mostdesirable. As will be discussed below, an antenna tuning circuit may beused to alter the effective electrical length of an antenna by loadingit with capacitance or inductance. The split-can antenna is especiallyadvantageous in this respect as compared with conventional wire antennasbecause it is physically wide and possesses a greater bandwidth. Anantenna with a greater bandwidth is easier to tune and is usable over agreater range of frequencies once it is tuned. A larger antennabandwidth also allows a higher data rate and minimizes the risk oflosing communications due to frequency drift.

[0013]FIG. 1 shows an exemplary implantable medical device 100 with adipole antenna suitable for radiating and receiving far-fieldelectromagnetic radiation formed by respective halves of the devicehousing 101 a and 101 b. The device housing is metallic and containstherapy circuitry TC1 for providing particular functionality to thedevice such as cardiac rhythm management, physiological monitoring, drugdelivery, or neuromuscular stimulation as well as circuitry RFC1 forproviding RF communications. One or more therapy leads 310 are connectedto the therapy circuitry contained within the housing by means of aheader 103 with feedthroughs located therein for routing the therapyleads to the appropriate internal components. The two housing portions101 a and 101 b are separated by a layer of insulating material 102.FIG. 2 shows an alternate embodiment in which the header is made ofdielectric material and is interposed between the two housing portions101 a and 101 b, thus also serving to separate the two legs of thedipole antenna. In either embodiment, the two housing portions 101 a and101 b are hermetically sealed with a minimum number of feedthroughsbetween them. A battery B1 is used to supply power to the electroniccircuitry within the housing. If the battery alone is contained withinone of the housing portions, then only two feedthroughs are neededbetween the two housing portions, one for each battery terminal.Alternatively, the battery and the RF circuitry can be placed in onehousing portion, with the rest of the device circuitry contained in theother portion. This shields the sensitive therapy circuitry from thevery noisy RF circuitry.

[0014]FIG. 3 is a block diagram of an exemplary implantable cardiacrhythm management device utilizing a split-can dipole antenna forradio-frequency telemetry. In the figure, only one therapy lead 310 isshown but it should be understood that a cardiac rhythm managementdevice may use two or more such leads. A microprocessor controller 302controls the operation of the therapy circuitry 320 which includessensing and stimulus generation circuitry that are connected toelectrodes by the therapy leads for control of heart rhythm and RF drivecircuitry 330 for transmitting and receiving a carrier signal at aspecified frequency modulated with telemetry data. The conductors of thetherapy lead 310 connect to the therapy circuitry 320 through a filter321 that serves to isolate the circuitry 320 from any RF signals thatmay be picked up by the lead. The filter 321 may be a low-pass filter ora notch filter such as a choke. The RF drive circuitry 330 includes anRF transmitter and receiver that are connected by a transmit/receiveswitch 333 to the dipole antenna formed by the housing portions 101 aand 101 b. The microprocessor 302 outputs and receives the datacontained in the modulated carrier generated or received by the drivecircuitry 330.

[0015] In this embodiment, the RF drive circuitry 330 is connected tothe dipole antenna through an antenna tuning circuit which loads theantenna with a variable amount of inductance or capacitance to therebyadjust the effective electrical length of the antenna and match theantenna impedance to the impedance of the transmitter/receiver. In thismanner, the reactance of the antenna may be tuned out so that theantenna forms a resonant structure at the specified carrier frequencyand efficiently transmits/receives far-field radiation. The tuningcircuit in this embodiment includes a balun transformer 400 and avariable capacitor 402 for loading the antenna with an adjustable amountof reactance. The balun transformer drives the two housing portions 180degrees out of phase and thus also serves to convert between thesingle-ended signal generated or received by the transmitter/receivercircuitry and the differential signal generated or received by theantenna. The balun transformer 400 also acts as a high-pass filter whichblocks low frequency energy from being passed to the RF circuitry suchas may be generated when the housing is used as an electrode indelivering electrostimulation with a monopolar lead.

[0016] Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. An implantable medical device, comprising: ahousing for containing electronic circuitry; first and second conductiveportions of the housing separated by an insulator; and, circuitry withinthe housing connected to the first and second housing portions fortransmitting or receiving a modulated radio-frequency carrier at aspecified carrier frequency, wherein the first and second housingportions constitute a dipole antenna.
 2. The device of claim 1 whereinthe first and second housing portions are opposite halves of the housingseparated by a layer of insulating dielectric material.
 3. The device ofclaim 1 wherein the first and second housing portions are oppositehalves of the housing separated by a header compartment made ofinsulating dielectric material.
 4. The device of claim 1 furthercomprising therapy circuitry and wherein the therapy circuitry and thetransmitter/receiver circuitry are contained in different housingportions.
 5. The device of claim 3 further comprising a batterycontained in one housing portion apart from other circuitry in thedevice.
 6. The device of claim 3 further comprising a battery andwherein the battery and transmitter/receiver circuitry are contained inone housing portion apart from other circuitry in the device.
 7. Thedevice of claim 1 further comprising a battery contained in one housingportion apart from other circuitry in the device.
 8. The device of claim1 further comprising a battery and wherein the battery andtransmitter/receiver circuitry are contained in one housing portionapart from other circuitry in the device.
 9. The device of claim 1wherein the dimensions of the first and second housing portions are suchthat a significant portion of radio-frequency energy delivered to theantenna at the specified carrier frequency is emitted as far-fieldradiation.
 10. The device of claim 1 wherein the electrical length ofthe antenna is approximately one-half wavelength or greater of theradio-frequency carrier at the specified frequency.
 11. The device ofclaim 1 further comprising an antenna tuning circuit for matching theimpedance of the antenna to the transmitting/receiving circuitry at aspecified carrier frequency by loading the antenna with inductance orcapacitance.
 12. The device of claim 11 wherein the tuning circuitcomprises a balun transformer for converting between a single-endedsignal generated or received by the transmitter/receiver circuitry and adifferential signal generated or received by the antenna.
 13. The deviceof claim 11 further comprising a variable capacitor for adjusting theresonant frequency of the antenna.
 14. The device of claim 1 wherein thedevice is a cardiac rhythm management device having rhythm controlcircuitry electrically connected to one or more electrodes adapted fordisposition within or near a heart by one or more therapy leads.
 15. Thedevice of claim 14 further comprising a filter connected to a therapylead for blocking radio-frequency signals from the rhythm controlcircuitry.
 16. The device of claim 15 wherein the filter is a notchfilter.
 17. A method for transmitting and receiving radio-frequencysignals in an implantable medical device, comprising: transmitting orreceiving a modulated radio-frequency carrier at a specified carrierfrequency to or from a dipole antenna formed by first and secondconductive portions of a housing; and, emitting a significant portion ofradio-frequency energy delivered to the antenna at the specifiedfrequency as far-field radiation.
 18. The method of claim 17 furthercomprising matching the impedance of the antenna to thetransmitting/receiving circuitry at a specified carrier frequency byloading the antenna with inductance or capacitance using an antennatuning circuit.
 19. The method of claim 17 further comprising convertingbetween a single-ended signal generated or received by thetransmitter/receiver circuitry and a differential signal generated orreceived by the antenna with a balun transformer.
 20. The method ofclaim 18 further comprising adjusting the resonant frequency of theantenna to a specified carrier frequency with a variable capacitor.